Chapter 6 -- Newton's Third Law: Action and Reaction

For every action, there is an equal and opposite reaction.

The basics:

• In every interaction, there is a pair of forces acting on the two interacting objects.
• The size of the forces on the first object equals the size of the force on the second object.
• The direction of the force on the first object is opposite to the direction of the force on the second object.
• Forces always come in pairs - equal and opposite action-reaction force pairs.

Real life examples of action-reaction force pairs:
A fish uses its fins to push water backwards. But a push on the water will only serve to accelerate the water. Since forces result from mutual interactions, the water must also be pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction force. Action-reaction force pairs make it possible for fish to swim.

Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. Since forces result from mutual interactions, the air must also be pushing the bird upwards. The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for birds to fly.

Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards. Since forces result from mutual interactions, the road must also be pushing the wheels forward. The size of the force on the road equals the size of the force on the wheels (or car); the direction of the force on the road (backwards) is opposite the direction of the force on the wheels (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for cars to move along a roadway surface.

Check Your Understanding:
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/newtlaws/u2l4a.html

Outline of Chapter 6 from Physics Text Book

Interactive forces, 0 net force

6.1 -- Forces and Interactions
A force is a push or pull
A force is not a thing in itself, but an interaction
Interaction -- Mutual action between one thing and another
Examples:
Interaction between hammer and nail

6.2 -- Newton's Third Law
Two forces: Action and reaction
Coparts of a single interaction
Neither force exists without the other
Equal in strength, opposite in direction
In every interaction, the forces always occur in pairs
Examples:
You interact with the floor when you walk on it
When swimming, you interact with the water
Car tires interact with the road

6.3 -- Indentifying Action and Reaction
One object (A) interacts with another object (B)
Action: A exerts force on B
Reaction: B exerts force on A
Other examples:
Action: Tire pushes road
Reaction: Road pushes tire
Action: Rocket pushes gas
Reaction: Gas pushes rocket
Action: Earth pulls ball
Reaction: Ball pulls earth

6.4 -- Action and Reaction on Different Masses
When a rifle is fired, there is an interaction between the rifle and the bullet. The force the rifle exerts on the bullet is exactly equal and opposite to the force the bullet exerts on the rifle, so the bullet shoots through the air (large acceleration & small mass) and the rifle "kicks" (small acceleration & large mass).

F= action and reaction forces
m= Mass of rifle
m= mass of bullet
a=acceleration of rifle
a=acceleration of bullet
Bullet: F/m = a
Rifle: F/m = a

6.5 -- Do Action & Reaction Forces Cancel?
Why it might be initially confusing:

• Actions & Reactions are equal and opposite.
  And if -16 + 16 = 0
  Shouldn't there be a net value of zero when considering
  The equal and opposite forces?
• That's not quite all there is to consider when answering...
  

Easy & Simple:
* Basically, you have to consider all of the systems
-*Internal
-*External

Reminder: *Newton's Third Law:

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.


*Important statements from 6.5:
- "If action and reaction forces are internal to a system, they cancel each other and produce no acceleration of the system."

6.6 The Horse-Cart Problem
AKA "Horse Sense" (Text 81)

• Context of the Problem: There is a farmer with a cart and a horse who needs to get to the market by moving forward. Horse argues that if he pulls on the cart (Force 1) the cart will pull backwards on him (Force 2) with an equal and opposite force, resulting in a net force of 0 and no acceleration of the cart or the horse.
• * Consider the different systems in the problem…

1. Cart and Horse Pulling in Opposite Directions.

(F2)<-----> (P) (P) <-----> (F1)

Force 1 (horse pulling forward) is cancelled out because Force 2 (cart pulling back) is equal and opposite to the force exerted on it.
= Zero Net Value & No Acceleration


2. Cart Alone

• Has to overcome...
  • mass of cart
  • wheel friction
  • air resistance
• farmer knows that...
  • A(cart) = F(cart) / M(cart)

So it needs more force to move it forward.
[needs more force--->]

The farmer knows that if the horse moves forward so will the cart. He needs the horse to provide the force needed to pull the cart, regardless of the force exerted on the horse.

3. Horse Alone

• without the cart the horse moves forward easily
  • - How could it increase the force it uses to move forward in order to overcome the force with which the ground (FG) pulls the mass of the cart backward?

• The horse is able to move forward by interacting with the ground the same way we do every day. By pushing down against the ground, the horse is using the equal and opposite force that the ground exerts back against it to push its body forward.

4. The Entire Horse-Cart System

Cart Horse
(F2)<-----> (P) (P) <-----------------------------------> (F1)
------>( F1+ Fa)
(FG)<-------- ----------------> (Fa)
*friction of wheels *friction of ground

So by pushing its hooves (Force a) against the ground (Force G) the horse is able to use the equal and opposite force from the ground is pushing back against it to move forward with a greater force.
The force that the horse receives from the ground is used to increase the force it uses to pull the cart.
There's now an external force (ground) in addition to the internal forces (cart & horse).
= Positive Net Force, & Forward Acceleration

Net Forces:
On Farmer – P (pull from horse) - f (friction)
On Horse – F(force 1) - P (pull from cart)
On Ground – F (force G) - f (friction)

*In order to accelerate, an external force is needed.


Here's the Horse-Cart problem again with numbers to represent the forces…

<(F2)------------Cart ------------(P)> <(P)-----------Horse -----------(F1)>
- 12 N +12 N

<(FG)----Ground ----(Fa)>
+ 10 N
Net Force = - 12 N + 12 N + 10 N = 10 N


Another Example....
IF YOU WANT TO MAKE SURE YOU UNDERSTAND, READ OVER THIS SIMILAR EXAMPLE:

Imagine your car has stalled. Would you sit in your front seat, pushing against the dashboard in order to move it forward? Obviously, this action would not help you move your car forward (the reaction would just be the dashboard pushing back against your hands).

If you needed to move your car forward, you would get out of it first of all, then push against it with your hands as you pushed against the ground with your feet.

Do you see how this is like the horse-cart example now?

Pushing on the dashboard as it pushes against you is an internal force and there will be no net force in this system alone. (Like the first part of the horse-cart example.)

When you push against the ground, it pushes against you with an equal and opposite amount of force, which you use to push against the car. With this force your car will move along the road because a net force will be created. (Like when the horse used the ground to move the cart.)


6.7 -- Action Equals Reaction
The Wall Example – With every action force, an equal and opposite reaction force occurs.
-You can not hit the wall any harder than the wall can hit you back.
-If you decide to hit a wall, be warned that the wall will exert a force on your hand as hard as you exert on the wall. The wall will hit you as hard as you hit it.

-----> <----- Equal & Opposite Directions


The Paper and the Heavyweight Champ Example – You can only act on an object with as much force as it can react (equal & opposite) with on you
-You cant hit paper a piece of paper in the air with a 250 N force.
-Why? - Because the paper cannot hit you back with that amount of force.
- There is no action force without an equal and opposite reaction force!
- But if your heart is set on punching a piece of paper with 250 N of force, you could do so if you taped the paper to the wall and then hit it. The wall will hit you back with an equal and opposite amount of force.